This invention relates to the cutting of glass with a laser beam focused upon the glass to be cut, and items made of glass cut with a laser beam.
Glass and other crystalline materials, such as ceramics, are extremely fragile and difficult to work with due to the inherent characteristics of their crystalline structure. One of the more common problems encountered is cutting or shaping the material into pieces having predetermined shapes and sizes, especially shapes with non-linear edges, inside corners, and accurate reproductions of a given shape. Historically, glass was broken by scribing it on the surface along a line with a diamond-tipped scribe or diamond-tipped saw. This weakened the crystalline structure and, hopefully, the glass would break along the scribe line when an appropriate pressure was exerted to create a force at the scribe line. These breaks extended from edge to edge because it is extremely difficult, almost impossible, to control the length of a break or fracture or to terminate it at a predetermined location. In addition, impurities and discontinuities in the crystalline structure, as well as impacts or uneven pressure distribution, caused unwanted breaks, cracks, or deviations in the desired cut. And, once the glass was broken, it was beyond repair. The scribe and break technique also suffers the drawbacks of contamination of the glass from handling or coolant, excessive mechanical strain on the glass, high tooling forces, unpredictable fracturing, and dangerously sharp edges. Thus, the scribing and breaking of glass is fraught with uncertainty, is time consuming, requires a great amount of skill to avoid undue waste of material, and is labor intensive.
A still further drawback to the scribing and breaking technique that is of at least equal severity is the limitation to making linear edge to edge cuts. It is very difficult and time consuming to make "inside" corners or curved cuts. In order to make an inside corner or curved cut, one typically scribes the desired break line and then cross-hatches the surface of the glass to be removed with a matrix of closely spaced additional scribe lines. The cross-hatched glass is then carefully removed in very small pieces with a pliers-like "nipper". While this process may work with larger pieces of glass, it is extremely difficult with smaller ones or where the remaining glass forms a finger-like protrusion having little lateral support. Unfortunately, this method suffers all of the uncertainties aforenoted for scribing and breaking, and it also results in still higher failure rates and more waste of material, as well as producing an extremely ragged edge along the cut. As a result, intricate shapes can be made from a single piece of glass only with extreme difficulty, patience, and skill. A still further major drawback to the edge to edge limitation of cuts is that it prevents one from removing a portion of the glass from the interior of a sheet with an unbroken support piece or background piece remaining.
In an attempt to overcome these limitations, others have utilized lasers to scribe glass, thereby eliminating the costly diamond-tipped tools which suffer high wear rates due to the hardness of the glass or ceramic. The high power density and small focused spot of a laser beam offers many advantages for scribing and then breaking glass. Lasers often scribe at a higher rate than conventional tools and provide a very narrow uniform scribe line, which minimizes loss of material. Scribe lines as narrow as 0.1 mm. or less are typical. The high speed, narrow scribe line combination results in a narrow heat affected zone which may also be limited to a width on the order of 0.1 mm. if desired. The positioning accuracy of the scribe line is as good as that of the positioning mechanism, and maintenance of this accuracy is augmented by the almost non-existent tooling forces exerted by a laser. The glass is subjected only to the forces of the positioning mechanism for the glass. In addition, changes or alterations in the cutting pattern may be effected with a simple change in the positioning mechanism, or if the position of the glass or the laser beam is numerically controlled, a change in the controlling program. Such changes may affect the glass position and velocity, laser beam power output, laser beam wave output, width of the scribe line, etc. As an alternative to line scribing, one may drill a series of closely spaced holes that generally extend vertically downward through the material. However, neither scribing nor drilling cuts all of the way through the material or removes all of the material along a given path, and one must snap or break the material along the scribe line or drill line. Thus, since the material is not cut through, scribing is limited to straight line separations. It is not preferred for configurations with inside corners, curves or complex shapes, because the crystalline structure of the glass will probably result in a straight edge to edge break, regardless of the position or location of the scribe line.
In another attempt to overcome the numerous limitations of scribing by either tool or laser, others have utilized lasers to break glass by "controlled" fracturing. Brittle or crystalline materials such as glass may be cut by using a laser to rapidly heat the material in a small zone. This heating produces a mechanical stress which results in localized fracturing. If the fracture can be controlled, this technique may be used to cut glass by moving the glass with respect to the laser beam. Although in theory the fracture follows the beam path, suggesting that one may make such a fracture along any desired path, actual practice has shown that good control of the fracture has been obtained only at low speeds along a straight or a gently curving path. Higher speeds, as well as sharp curves or corners, have resulted in fractures which propagate without control. It has been reported that controlled fracturing has not been adopted in large scale operations.
Prior to applicants' invention, attempts to cut glass with a laser have been unsuccessful due to the intrinsic stress and fracture characteristics of the glass when it is subjected to intense localized heat. As a rule, glass cracks when heated non-uniformly. Accordingly, when glass is heated by an incident focused laser beam, the severe thermal stress can uncontrollably crack the glass. One proposed solution involving cutting glass with a laser wherein the glass was preheated to its annealing temperature (typically 950.degree.-1100.degree. F.), whereby cracking could not be sustained. This process required a specially designed furnace for heating the glass, including appropriate supports for the hot glass, and a separate furnace to slowly and uniformly cool the glass after cutting. This process involved vaporization of the entire glass thickness, which refers to decomposition of the glass rather than the usual meaning of phase change from a liquid or solid to a gas. The drawbacks of this process include a change of the color of the glass, a change in transparency in a region on each side of the cut, creation of a zone of gas bubbles trapped within the glass on each side of the cut, and an uncut transient section at the beginning of the laser path. Still further drawbacks include a high power requirement, not only for operation of the laser to achieve total vaporization, but also for preheating the glass and controlling the rate of cooling after the cut is made.